Apparatus for the manufacture of thermoplastic tubes



March 20, I s EVERETT APPARATUS FOR THE MANUFACTURE OF THERMOPLASTICTUBES Original Filed Feb, 27, 1946 2 Sheets-Sheet 1 FIGS 4' A I I g r l3 23 I Inventor Attorney S. J. EVERETT March 20, 1951 APPARATUS FOR THEMANUFACTURE OF IBERMOPLASTIC TUBES 2 Sheets-Sheet 2 Original Filed Feb.27, .1946

I nvenlor R o m E Q W J B R L E 0 M a m Patented Mar. 1951 UNITED STATESPATENTMOFFICE APPARATUS FOR THE MANUFACTURE OF THERMOPLASTIC TUBESSamuel James Everett, Thornton Heath,

England Original application February 27, 1946, Serial No.

650,419. Divided and this application Septem- -ber 19, 1947, Serial No.774,918. In Great Britain March 12, 1945 (oi. ii -'1') 7 tion, byshrinking a tube, while in a plastic condition on to a mandrel by meansof raised pressure acting, on the outside of the tube and introducingalubricant which is preferably alubricating oil, hetweenthe tube and themandrel to prevent the tube sticking to the mandrel. If necessary,oxidation of the oil is prevented by evacuating the air from the insideof the tube or by filling the tube with an inert gas such as nitrogen.The

mandrel is provided with an absolutely smooth surface made accurately tosize on to which the thermoplastic tube is moulded and the mandrel is ofa material having a co-efficient of thermal expansion greater than thatof the material of the tube. The result is that as the tube and mandrelcool the mandrel contracts away from the tube and the tube mayconsequently be readily separated from the mandrel. Such contractionoccurs longitudinally as well as radially of the tube and it is for thatreason that oil which prevents the tube from sticking to the mandrel isused. In the absence of oil, the thermoplastic tube is liable to crackor even to be fractured as a result of the longitudinal contraction ofthe mandrel relatively to the tube. Machine oil may be used but oil asused in vacuum pumps is preferred. Moreover graphite may be employed butonlyin small amounts as otherwise it interferes with the accuracy of thefinish of the interior of the tube. i iv .The novel. process may be emloyed in forming tapered or p'aralleltubes and tubes of circular ornon-circular cross-section. If a very long length of tube is to betreated, the mandrel may consistfo'f a plurality of lengths carefullysnig otted together to present a true and continuous surface. Thetube-to be treatedmaybe glass cane which has.been:.:subjected.to. apreliminary stretching In order that the invention may be clearlyunderstood and readily carried into effect, two examples of the novelprocess will now be deg, scribed in greater detail with reference to theaccompanying drawings, in which- Figure 1 is an axial section of theapparatus for carrying out the invention according to one method;

Figure 2 is a cross-section of the pressure casing containing the tubeand mandrel, taken on the line II-II in Figure 1;

Figure 3 is a side elevation partly in axial section of a furnace andassociated apparatus for carrying out a second method; and Figure l is adiagrammatic side elevation of the complete apparatus used for the saidsecond method. I'

- -In thetwo examples selected, the tubing treated is tubing of theglass sold under the registered trade mark Pyrex having a coefficient ofthermal expansion of 3.5x 10- and the mandrels employed are of carbonsteel having a considerably higher coefiicient of thermal expansion ofthe order of '10 l0 In Figures 1 and 2, the glass tube I after its lefthand end has been sealed at 2 is fitted on to a mandrel 3 which is ofsuch a size in crosssection as very nearly to fit the tube i so thatthere is no risk of the tube crumpling instead of shrinking when it iscaused to contract on to the mandrel 3. Oil is then introduced into thetube I to prevent the latter sticking to the mandrel 3 during theshrinking process.

In this example, the tube l and mandrel 3 are inserted in and supportedin a cylindrical steel pressure casing 4 which is renderedpressure-tight by means of a seal at its open end. This seal consists ofa flanged cap 5 slidably fitting over the casing t with rubber discs 6compressed between it and the end of the casing 4 by screws l drawingthe flange of the cap 5 towards aflflange 8 fitted to the casing' i.Thus the discs 6 are caused to spread inwardly and grip the tube 1 Airunder pressure is conducted to. the casing 4 through a connecting tube 9while air can be evacuated from the tube I through a connector Illleading to a vacuum pump.

The tube is heated in successive sections by the casing 4 being drawnfrom left to right in Figure ltthrough a furnace .11.. I This :is shownas a tubular electrically heated'furnacehaving a refractory liner l2closed by end walls !3 of asbestos board drawn together by threaded rods14 and nuts IS, the whole being clamped to two angle rings 16. Anelectric heating element ii is shown outside the liner l2 and is encasedin heat insulation l8 held in place by an outer tube l9.

The pressure in the casing i is raised to 120 lbs. per square inch andthe temperature in the furnace II preferably to about 650 C., althoughthe latter may be as high as 750 C. This increased temperature reducesthe time necessary to shrink the tube I but increases the risk of thetube I sticking to the mandrel 3.

After the casing 4 has been slowly moved through the furnace I I, it maybe allowed to cool to 350 C. and the tube I removed while the mandrel 3owing to the difference in thermal expansion between the mandrel andtube may be easily withdrawn from the tube I.

In Figure 3, the furnace II is of substantially the same construction asin Figure 1 except that extensions 20, 2I are secured axially atopposite ends to the walls I3 by screws not shown. It is intended thatthe glass tube I' containing the mandrel 3 shall be made to travelcontinuously from left to right in Figure 3 through the furnace I I andin order that pressure may be maintained in the furnace while the tube 5and mandrel 3 so travel, pressure seals 0:" rubber 22 are made to bearon the glass tube at the points of entry and exit. The air pressure issupplied to the interior of the furnace through the connecting pipe 23from an air pump. An air admission cook 24 and a pressure release cock25 are fitted. In this case, the tube I after it leaves the furnace IIis cooled in the extension 2| which is made to serve as a water jacketand is fitted with a water inlet 26 and water outlet 21.

The tube I is passed through the hot zone of th furnace II by feedingmeans at both ends which may operate at the same or different speeds sothat the tube I may be kept at the same weight per foot length duringits treatment or may be stretched or compressed. The operating mechanismfor the feeding means is shown diagrammatically in Figure 4. Itcomprises a chuck 28 gripping the tube I at its forward end and asimilar chuck 29 gripping the tube before it enters,

the furnace II. The chucks 28, 29 are carried on brackets 30 releasablyengaging lead screws 3i, 32 journalled in the frame members'33, 34 andprevented from axial movement. These lead screws are rotated by anelectric motor 35 driving a pulley 36 through a belt 3!. The pulley 36carries a worm engaging a worm wheel 38 which is connected to the leadscrew 3I through doublecone pulleys 39, 4B and a further belt 4! andpulleys 42, 43. The lead screw 32 is driven at an adjustable speedthrough a stepped pulley 44 and belt 45.

In Figures 3 and 4, also the tube I may be r sealed at its left hand endand evacuated at its right hand end through a flexible pipe or an inertgas may be continuously passed through it to prevent oxidation of theoil in the tube I.

The glass tube I is heated up to about 700 C. in the furnace II in whicha'pressure of 3 atmospheres upwards is maintained depending on thethickness of the glass tube I.

By the use of the present invention, the glass tube may be provided withtransfers or ceramic markings on the surface which are thoroughly burnedin the furnace and are not removed by boiling in alkalies or acids' Iclaim:

1. An apparatus for forming a thermo-plastic tube with a fine qualityinternal surface comprising, in combination, a furnace including'aheating element for generating sufiicient heat to soften said tubing, agas-tight chamber within the heating zone of said furnace, a removablemandrel located within a piece of tubing to be formed, means for feedinginto said chamber said thermo-plastio tubing containing said removablemandrel, a flexible pressure seal at one side of said chamber for makinga gas-tight seal with the thermo-plastic tubing at it enters thechamher, a source of fluid under a pressure of at least threeatmospheres connected to said chamber for communication with the spaceabout the outside of said tubing to compress said tubing onto saidmandrel while in softened condition, means for withdrawing said tubingfrom said chamber, a flexible pressure seal at the opposite side of saidchamber for making a gas-tight seal with said tubing as it leaves saidchamber, and means surrounding said tubing at the withdrawal side ofsaid chamber for passing a coolant in close proximity to said tubing asit leaves said chamberto lower the temperature of the tubing quickly to9i point where said mandrel can be removed safely; 2. An apparatus forforming a thermo-plastic tube with a fine quality internal surfacecompris ing, in combination, a furnace including a heating element forgenerating sufficient heat to soften said tubing, a gas-tight chamberwithin the heating zone of said furnace, a removable mandrellocatedwithin apiece of tubing to be formed, means for feeding into saidchamber said thermo-plastic tubing containing said removable mandrel, aflexible pressure seal'at one side of said chamber for makingiagas-tight seal with the thermo-plastic tubing as it enters the chamber,a source of fluid under a pressure of at least three atmospheresconnected to said chamber for communication with the space about theoutside of said tubing to compress said tubing onto said mandrel whilein softened condition, means for withdrawing said tubing from saidchamber, flexible pressure seal at the opposite side of said chamber formaking a gas-tight seal with said tubing as it leaves said chamber,means surrounding said tubing at the withdrawal side of said chamberfdr'passing a coolant in close'proximityto said tubing as it leaves saidchamber to lower the temperature of the tubing quickly'td a point wheresaid mandrel can' be removed safely, said feeding 'andwithdrawing meansin eluding gearing means for varying the rates of feed and withdrawal ofsaid tubing, whereby the weight per unit length of the finished tube maybe varied, while a uniform relation between the interior of the tube andthe mandrel is maintained by said fluid pressure. SAMUEL JAMES EVERETT.g

REFERENCES CITED The following references are. of record in the file ofthis patent:

' UNITED STATES. PATENTS

1. AN APPARATUS FOR FORMING A THERMO-PLASTIC TUBE WITH A FINE QUALITYINTERNAL SURFACE COMPRISING, IN COMBINATION, A FURNACE INCLUDING AHEATING ELEMENT FOR GENERATING SUFFICIENT HEAT TO SOFTEN SAID TUBING, AGAS-TIGHT CHAMBER WITHIN THE HEATING ZONE OF SAID FURNACE, A REMOVABLEMANDREL LOCATED WITHIN A PIECE OF TUBING TO BE FORMED, MEANS FOR FEEDINGINTO SAID CHAMBER SAID THERMO-PLASTIC TUBING CONTAINING SAID REMOVABLEMANDREL, A FLEXIBLE PRESSURE SEAL AT ONE SIDE OF SAID CHAMBER FOR MAKINGA GAS-TIGHT SEAL WITH THE THERMO-PLASTIC TUBING AT IT ENTERS THECHAMBER, A SOURCE OF FLUID UNDER A PRESSURE OF AT LEAST THREEATMOSPHERES CONNECTED TO SAID CHAMBER FOR COMMUNICATION WITH THE SPACEABOUT THE OUTSIDE OF SAID TUBING TO COMPRESS SAID TUBING ONTO SAIDMANDREL WHILE IN SOFTENED CONDITION, MEANS FOR WITHDRAWING SAID TUBINGFROM SAID CHAMBER, A